In recent years automobile manufacturers have offered as optional equipment rear windows which can be defrosted and/or defogged by use of an electrically conductive grid permanently attached to the window. In order to defrost quickly the circuit must be capable of supplying large amounts of power from a low voltage power source, for example, 12 volts. Furthermore, the lines of the conductive grid must be sufficiently narrow in order to maintain visibility through the rear window.
Heretofore, the materials used for the preparation of window defogging grids have mostly been thick film silver conductors which are prepared from paste comprising finely divided silver powder particles and glass frit dispersed in an organic medium. In a typical application a paste containing by weight 70% silver powder, 5% glass frit and 25% organic medium is screen printed through a 180 Standard Mesh Screen onto a flat, unformed glass rear window. The printed composition is dried for two minutes at about 300.degree. C. and the entire element is then fired in air for from 7 to 10 minutes at 650.degree. C. After firing the softened glass is shaped by pressing into a mold and then tempered by rapidly cooling. During the firing cycle the organic medium is removed by evaporation and pyrolysis. The glass and silver are sintered to form a continuous conductive path with the glass acting as binder.
The silver compositions currently used yield upon firing resistances of from 2 to 15 milliohms per square. The resistance requirements vary according to the size of the conductive grid and hence the window. Conductors for large window areas need more electrical current because they have more area to defrost and therefore have much lower resistance requirements. Thus, the larger rear window area is typical of full sized cars require as little as 2 milliohms per square resistance, whereas the relatively small rear window area which is typical of compact cars can utilize compositions having resistance of as high as 15 milliohms per square.
Because of the current trend toward smaller cars the automotive industry anticipates a decline in the need for very low resistance silver compositions (2 to 4 milliohms per square) and forecasts suggest that the future resistance requirements will be for compositions of from 3 to 8 milliohms per square.
Such resistance requirements for defoggers are easily met by noble metal conductors, particularly silver, which is currently the most widely used conductor material. However, silver conductors are quite expensive, thus, there is a need for base metal conductor compositions which can meet the resistance and other physical requirements for defogger compositions. Unfortunately, the prior art base metal conductors do not sufficiently meet these criteria. For example, U.S. Pat. Nos. 4,148,761 and 4,207,369 are directed to electroconductive materials containing 0.25-30% by weight silicon, 20-90% aluminum and 10-50% glass having a melting point below 660.degree. C. The electronconductive materials are prepared by conventionally firing a mixture of aluminum metal powder silicon metal powder and glass frit. These compositions have been shown to have sheet resistances of from 9 to 18 milliohms per square. Thus, they are not quite good enough for future defogger requirements even though they are relatively inexpensive.
U.S. Pat. Nos. 4,122,232 and 4,148,761 are concerned with the prevention of oxidation of base metals, particularly nickel, upon firing conductor pastes comprising powdered base metal, glass frit and liquid organic medium. Boron powder is added to the composition to reduce oxidation of the base metal upon firing. The resultant conductors are shown to have resistances of as low as 100 milliohms per square. In addition, it has been shown that such boron-containing compositions give defoggers which are highly moisture sensitive. Thus they are further removed from acceptability for use in defogger compositions when the resistance requirements are at a low level of 8 milliohms per square or less.